Generally, if the depth of the seating surface of a chair is too large or too small as compared with the build of a sitter, the sitter is forced to assume an unnatural posture. In this respect, there is conventionally known a chair of the type which includes a seat portion and a backrest portion and which is constructed to be capable of adjusting the depth of the seat portion that can substantially support the back of a sitter according to the build of the sitting person by moving either the seat portion or the backrest portion back and forth substantially horizontally.
Such a chair, however, has an inconvenience that when the backrest portion is moved forward or the seat portion is moved backward, an rear end portion of the seat portion protrudes rearwardly beyond the backrest portion, making the appearance of the chair very bad. A chair of the type which is free of a backrest portion or which is incapable of moving its seat portion or backrest portion back and forth, inherently cannot adjust the depth of the seat portion based on the aforementioned construction and, therefore, a sitter is forced to sit thereon in an uncomfortable posture if the specifications of the chair are not suited to the build of the sitter. To avoid such an inconvenience, a chair suited to the build of an individual sitter must be obtained (first problem).
Also, there is conventionally known a chair of the type constructed to allow its seat portion to move back and forth with a rear end portion thereof gradually rising so that the depth of the seating surface can be adjusted according to the build or posture of a sitter. In the seat portion of such a chair, usually the reverse side of a seat body having a cushioning property is substantially entirely covered with a relatively rigid shell formed from a resin or the like.
Such a construction, however, gives rise to an inconvenience that the rear end portion of the seat body gradually rising rearwardly does not curve smoothly due to the seat body covered with the rigid shell that does not bend so much and, hence, wrinkles of projection and depression shape are formed on an upper surface of the seat body at the boundary between a substantially horizontal front end side and a gradually rising rear end side, with the result that the seat body, which essentially must be soft to a certain degree, becomes hard at the wrinkled portion, thus imparting a sense of incongruity to the sitter at his or her buttocks, hence making the chair uncomfortable to sit.
With a view to eliminating such an inconvenience, a chair has been devised such that the reverse side of a seat body including its rear end portion adapted to rise gradually is substantially entirely covered with a flexible shell to avoid the formation of hard wrinkles of projection and depression shape on the seat body. Such a chair, however, is not necessarily comfortable to sit because the shell deforms flexibly along a seat support even at a portion supporting the thighs of the sitter and, hence, the sitter feels the hardness of the seat support constructed of a metal frame or the like at his or her thighs through the shell and cushion (second problem).
Practical use has recently been made of chairs provided with various mechanisms including a backrest inclining mechanism, a rocking mechanism, a fore-and-aft seat position adjusting mechanism, and like mechanism for sitters to assume a proper posture at work, a resting posture or a refreshing posture selectively.
When attention is paid to the fore-and-aft seat position adjusting mechanism among such mechanisms, a conventional one is constructed to allow the seat portion to move back and forth relative to the seat support and to be locked at a desired position and, hence, a manipulating section for operating this mechanism is usually located on the seat support side along with manipulating sections for operating the rocking mechanism and the like.
However, the fore-and-aft movement of the seat portion causes the position of the manipulating section relative to the seat portion to change. This may result in an inconvenience that when the sitter wants to operate the fore-and-aft seat position adjusting mechanism while keeping his or her sitting posture, the sitter has a difficulty in catching the manipulating section properly with his or her fingers if the sitter puts his or her hand on the seat portion, or the sitter is likely to lose his or her balance due to the support for the sitter's body becoming unstable if the sitter puts his or her hand on the manipulating section. A chair of the type constructed to move the seat portion back and forth with the help of manipulating power of a human putting his or her hand on the seat portion, in particular, requires firm grasping of the seat by the sitter, so that the sitter has a difficulty in catching the manipulating section with his or her fingers and hence is forced to perform a more difficult manipulation (third problem).